1. Field of the Invention
The present invention relates to an exhaust gas purifying apparatus for an internal combustion engine, and particularly to an apparatus having a selective reduction catalyst which reduces NOx in exhaust gases under existence of a reducing agent.
2. Description of the Related Art
The NOx removing device for removing NOx in the exhaust gases using the selective reduction catalyst is shown, for example, in Japanese Patent Laid-open No. 2009-209765 (JP-'765). This NOx removing device is provided with the selective reduction catalyst and an aqueous urea solution (hereinafter referred to as “urea solution”) supply device for supplying urea solution as a reactant for generating a reducing agent, to the upstream side of the selective reduction catalyst. According to this device, a reducing agent supply amount is controlled by controlling an amount of the urea solution supplied to the selective reduction catalyst.
Even if an amount of the urea solution is maintained at a constant value, a suitable supply amount of the reducing agent changes corresponding to a change in the urea concentration of the urea solution. Therefore, it is necessary to monitor the urea concentration. Japanese Patent Laid-open No. 2007-163177 (JP-'177) and Japanese Patent Laid-open No. 2005-337969 (JP-'969) disclose techniques in which a sensor (urea concentration sensor) for detecting an urea concentration is provided in an urea solution tank which stores the urea solution.
Further, JP-'765 shows a method for estimating the urea concentration according to the freezing point of the urea solution without using an urea concentration sensor. Specifically, the urea solution which is frozen by reducing a temperature of the urea solution in the tank and the freezing point of the urea solution is measured by detecting a temperature at which the frozen urea solution melts.
Further, Japanese Patent Laid-open No. 2008-546968 (JP-'968) shows an exhaust gas purifying apparatus in which ammonia gas is supplied to the selective reduction catalyst as a reducing agent. According to this apparatus, the ammonia gas supply amount is controlled so that a ratio of ammonia to NOx becomes optimal with suppressing an outflow of ammonia to the downstream side of the selective reduction catalyst.
As shown in JP-'177 and JP-'969, the apparatus using the urea concentration sensor invites a cost increase and an increase in weight and size. Further, the urea solution in the tank vibrates when the vehicle runs and air bubbles may be generated by the vibration. Therefore, detection accuracy of the urea concentration may deteriorate due to the vibration and the generation of air bubbles.
Further, the method shown in JP-'765 has a problem described below. FIG. 41 shows a relationship between the freezing point TSOL and the urea concentration CUR. As apparent from FIG. 41, it is correctly detectable that the urea concentration CUR is equal to the normal concentration CUR0 (32.5%) (TSOL=TSOL0). However, when the freezing point TSOL, for example, is equal to TSOL1, the urea concentration CUR may be eqaul to the concentration CUR1H higher than the normal concentration CUR0 or to the concentration CUR1L lower than the normal concentration CUR0. Therefore, it cannot be determined which is the correct urea concentration. In general, it is considered that the urea concentration CUR changes in the decreasing direction. However, there may be a case where the urea concentration changes in the increasing direction depending on the using condition, or a case where the urea solution outside the standard is used. Accordingly, the urea concentration may incorrectly be determined by the method shown in JP-'765.
Further, JP-'765 discloses that an abnormality warning is performed when the estimated urea concentration is equal to or lower than the threshold value. However, the urea solution supply amount control based on the estimated urea concentration is not shown in JP-'765.
Further, according to the apparatus shown in JP-'968, a flow rate of the ammonia gas is detected by a flow rate sensor, and the flow rate control valve is controlled so that the detected ammonia gas flow rate coincides with its command value. Accordingly, there is a possibility that accuracy of the ammonia supply amount control may be reduced due to differences in characteristic of the flow rate sensors or the flow rate control valves, or due to aging deterioration of the flow rate sensor or the flow rate control valve.